In recent years there has been a great deal of interest and progress in the integration of implantable medical devices such as defibrillators and pacemakers. For the purpose of this application, "defibrillation" is used in a broad sense, as including the application of relatively high energy and high voltage shocks to the heart to terminate tachyarrhythmias including fibrillation and malignant tachycardias. Similarly, "pacing" is used in a broad sense, as including the application of relatively low energy and low voltage pacing pulses to maintain an adequate heart rate or to break a tachycardia by stimulating the patient's heart. One traditional approach to combining pacing and true bipolar sensing electrodes in a defibrillation lead is to provide a ring electrode located between the pacing tip electrode and the defibrillation electrode where the ring electrode is dedicated exclusively to sensing the heart's electrical activities. The space required for this ring electrode forces the defibrillation electrode to be set back from the RV apex, and, because of the size limitations of the right ventricle, decreases the length available for the defibrillation electrode.
However, in the context of endocardial ventricular leads, it would be desirable to provide an electrode, or electrode pair, for sensing adjacent the ventricular apex, while still providing an electrode which also is located as close to the apex as possible.
Exemplary attempts to accomplish such objective are described in U.S. Pat. No. 5,336,253, to Gordon et al., and U.S. Pat. No. 5,342,414 to Mehra, both of which are incorporated herein by reference in their entirety. The Gordon patent describes a combined pacing and cardioversion lead system with internal electrical switching components for unipolar or bipolar sensing of electrograms, pacing at normal pacing voltages and cardioversion or defibrillation. In bipolar embodiments, a ring electrode is coupled through the internal switching circuitry to a large surface area cardioversion electrode. In these embodiments, pacing and sensing are accomplished through a pair of conductors extending through the lead body to the tip and ring electrodes. When cardioversion shocks are delivered to the ring electrode, cardioversion energy is also directed to the cardioversion electrode through the operation of the switching circuitry in response to the magnitude of the applied cardioversion pulse.
However, the lead system disclosed in the Gordon patent uses discrete and non-programmable internal switching components, such as the zener diodes in the arrangement of FIG. 3, or the surge suppressor and the resistor in the arrangement of FIG. 4. These internal switching components appear to indiscriminately and automatically connect the ring electrode to the cardioversion electrode upon the application of a cardioversion pulse exceeding a predetermined magnitude. As a result, the lead system described in this patent lacks the required flexibility to adapt the application of the cardioversion shocks to specific cardioversion conditions in progress.
Wherefore, it would be highly desirable to have a new lead switching matrix for use in an implantable cardiac stimulator for the detection and management of cardiac arrhythmias. It would also be desirable to have a new method for activating a true bipolar sense electrode to deliver high voltage therapies.
The transvenous defibrillation lead described in the Mehra patent is directed towards optimizing the size, spacing and location of the electrodes, and more specifically towards providing a bipolar sensing pair of electrodes having adequate interelectrode spacing to insure appropriate sensing of cardiac depolarization, while still allowing the placement of the electrode as close to the distal end of the lead body as possible. The lead includes a helical electrode, extending distally from the lead body, for use as the active electrode in cardiac pacing and for use in sensing cardiac depolarizations. A ring tip electrode or a cylindrical ring electrode is located at or adjacent to the distal end of the lead body and provides the second electrode for use in sensing depolarizations. The helical electrode is insulated from the point it exits the lead body until a point adjacent to its distal end. The defibrillation electrode is mounted with its distal end closely adjacent to the distal end of the lead body, such that its distal end point is within one centimeter of the distal end of the lead body.
The leads described in the foregoing Gordon and Mehra patents do not provide for integrated bipolar sensing, wherein sensing is carried out between the cardioversion electrode and the tip electrode. One feature that distinguishes integrated bipolar sensing and true bipolar sensing is that integrated bipolar sensing lacks an electrode dedicated solely to bipolar sensing in conjunction with the pacing tip. Typically, in an integrated bipolar electrode, the same electrode used for bipolar sensing in conjunction with the pacing tip is used to deliver defibrillation or cardioversion therapies. There are two potential problems with integrated bipolar electrodes. First, because the integrated electrode must be large for efficient delivery of defibrillation or cardioversion energy, it may reduce the resolution of the sensed signal due to spatial averaging of the different potentials within the heart. Secondly, the integrated electrode serves also as a defibrillation electrode and is likely to have substantial residual charge at its interface after a defibrillation therapy pulse. The residual charge or polarization of the electrodes results in less accurate sensing immediately after therapy. The true bipolar sense electrode should not be subject to these potential problems. The size of the true bipolar electrode is not governed by the need for efficient energy delivery during therapy and can be optimized for sensing. Additionally, because a negligible current flows across the electrode tissue interface, there is no build-up of charge or polarization at the interface, enabling the accurate measurement of endocardial signals immediately following therapy. However, a drawback with true bipolar sensing exists because the sense electrode in a true bipolar lead is located adjacent to the pacing electrode, and thus the cardioversion electrode is generally positioned further away from the apex of the heart, thus disadvantageously reducing the delivered therapeutic energy.
Therefore, it would be desirable to have a new lead which permits the optimal delivery of defibrillation and cardioversion energies, and the minimization of poor sensing due to polarization effect. It would also be desirable to optimize the electrode functionality without the complexity and dimensional constraints of circuit elements located within the lead.